This invention relates to the preparation of compounds or alloys particularly those used in semiconductors such as gallium phosphide in general and more particularly to an improved mehtod of detecting a leak in the reaction ampoule in which the preparation is taking place and shuting down the process upon such detection.
In the preparation of compounds and alloys of this nature such as gallium phosphide and which will typically include one component which is highly volatile reactive component the preparation is very often carried out in a closed system, the reaction is carried out within a closed quartz vessel referred to as a reaction tube or an ampoule. This may be necessary for reasons of purity and because of the inertness of the vessel materials being used. During the reaction in preparing the compound or alloy a vapor pressure will be built up within the ampoule because of the highly volatile component. Because of this it is desireable and sometimes necessary to place the reaction tube within an autoclave or pressure vessel and to apply to its outside walls a pressure which corresponds to the internal pressure so that the differential pressure acting on the walls of the reaction ampoule will be minimized.
A procedure of this nature has been previously described by Frosch and Derick in the Journal of Electrochemical Society, Vol. 108, pg. 251 (1961). In the disclosed process for the synthesis of polycrystalline gallium phosphide from the two elements the synthesis is carried out in a closed reaction tube or ampoule with the outside of the tube having a counter pressure applied thereto which is approximately the magnitude of the internal ampoule pressure. This pressure is obtained through the use of an inert gas within a pressure vessel and permits using quartz ampoules which are commercially available, i.e. quartz ampoules of relatively small wall thickness.
Improved methods and apparatus for carrying out such a process and greater detail regarding the process is given in Application Ser. Nos. 559,015, and 559,016, and 559,014 all filed on Mar. 17, 1975 and assigned to the same assignee as the present invention.
The fabrication of the quartz ampoule in such a process can be simply carried out. The charged ampoule, i.e. the ampoule after having the elements placed therein, can be sealed off using an oxygen-hydrogen torch. After placing the two elements in the ampoule a sealing cap or sealing block is inserted after which the ampoule is evacuated to a pressure of about 10.sup..sup.-5 Torr. The wall of the ampoule is then fused to the sealing cap using the oxygen-hydrogen torch. This is a well known sealing process and results in a vacuum tight and pressure tight closure of the ampoule. The types of ampoules which may be used and additional information regarding the sealing process is given in application Ser. No. 561,342 filed on even date herewith and assigned to the same assignee as the present invention.
Although excellent seals are obtained there are occasions when the seals do not withstand the stress at high temperature which can last for several hours during the synthesis. They do not remain tight, since, for example, thermal stresses in the quartz glass, caused by the sealing process, cannot be annealed by tempering, since this would represent a considerable cost factor. As a result, should a break occur in the ampoule at its seal or in any other area the reaction element or elements which are volatile at the reaction temperature can escape from the ampoule and react, for example, with the heater windings of the resistance furnaces used in the apparatus or can condense at cool surfaces and lead to undesireable consequences such as the ignition of white phosphorus in air when the pressure vessel is opened. Furthermore, because the volatile reaction component escapes from the leaky ampoule, it is not available to carry out a complete reaction. As a result, when such leakage occurs the reaction should be immediately interrupted. However, it is not possible under normal circumstances to directly observe the seal and detect such cracks. Consequently, the damage can already have taken place before the leak is recognized. This has been found to be a particular problem in the preparation of polycrystalline AIII-BV compounds. In particular, compounds with one or more volatile components such as As, P, and S exhibit problems.
In view of this it is evident that there is a need to be able immediately to detect a leak in such a system and upon such detection shut down the system.